1. Field of the Invention
The present invention relates to an optical recording medium such as a DVD-R, and to a device and method for optical recording information onto the optical recording medium.
2. Detailed Description of the Related Art
In a high density optical recording medium such as a DVD-R, laser pulses are directed to the recording layer, made of organic dye or the like, in response to data to be recorded thereon such that marks having a length corresponding to the data are successively formed with a spacing disposed between the marks. To accomplish high density recording on such an optical recording medium, it is necessary to reduce the marks and the spacing disposed therebetween as much as possible.
Laser pulses are directed to the recording layer at a predetermined recording speed as mentioned above. The energy of the laser pulses is accumulated in the marks adjacent to each other along the direction of recording and a spacing disposed therebetween, causing thermal interference between a mark and the spacing.
For example, to avoid the thermal interference, the conventional DVD-R recording methods (strategies) include dividing laser pulses, directed to a DVD-R to form marks thereon, into a top pulse having a large pulse width and multi-pulses repeated at short time intervals, thereby preventing heat from being accumulated upon completion of irradiation of the laser pulses.
In addition to this, the recording strategy is changed because of the following reasons to thereby control the thermal interference.
For example, consider two combinations: a combination of a mark, a long spacing, and a mark; another combination of a mark, a short spacing, and a mark. In these cases, recording these combinations by the same recording strategy would cause different amounts of heat to be accumulated in each mark and different recording mark lengths to be produced in the same duration of laser pulse irradiation due to the difference in length of spacings disposed between the recording marks. This would ultimately lead to a difference in total jitter. Here, the total jitter means the jitter of a recording mark/spacing produced along the time axis and normalized with channel clocks.
Suppose that a mark has been formed and is followed by a short spacing. In this case, the amount of heat generated during the formation of the mark would affect the mark subsequent to the spacing, resulting in an elongated mark and a shortened spacing.
In the case of a DVD-R, such a thermal interference has a serious effect on a combination of a 3T spacing disposed between marks, the spacing having 3T (3 channel bit length) equal in length to the shortest mark of the DVD.
For this reason, such a recording strategy has been conventionally employed for recording in which the start of a laser top pulse subsequent to a predetermined short spacing is delayed so as to shorten the length of an emitted pulse than in other cases.
The mark subsequent to a 3T spacing or the shortest spacing would be seriously affected by the thermal interference. Therefore, such a laser pulse delay time for shortening the length of the mark and the amount of time reduction Q(ns) after the 3T spacing are effective to prevent total jitter.
As described above, Q can be used to form a mark subsequent at least to a spacing equal in length to the shortest mark provided for recording. In any combinations of a mark, the shortest spacing, and a mark or of a mark, a long spacing, and a mark, this allows the lengths of the last marks (or spacings) to be aligned with each other irrespective of the length of the spacings, thereby reducing the total jitter.
It has been conventionally practiced upon recording to determine the optimum Q value based on an asymmetry obtained by a signal after the recording or on a fixed value employed for roughly expecting the power of the asymmetry. However, different recorders provide different expectation values and thus provide different asymmetries upon recording the same optical recording medium, thereby making it impossible to determine the optimum Q value in a fixed manner.